EP3204825B1 - Optical system for producing lithographic structures - Google Patents

Description

The present patent application takes the priority of the German patent application DE 10 2014 220 168.3 to complete.

The invention relates to an optical system for lithographic structure production. Furthermore, the invention relates to a method for determining relative coordinates of a position of a writing field relative to a position of a Vorschaufeldes in such an optical system and a method for lithographic structure production with such an optical system.

An optical system of the type mentioned is known from the US 2013/0221550 A1 wherein a projection lens is disclosed with an objective lens for guiding a pattern-generating writing light beam onto a substrate carried by a substrate holder. As deflecting device for deflecting a writing focus of the writing light beam, a scanning mirror is used. An optical system of the type mentioned is also known from the US 2013/0 223 788 A1 , From the DE 103 15 086 A1 For example, a method and apparatus for aligning semiconductor wafers in semiconductor manufacturing are known. The lithography system disclosed therein comprises an exposure unit, a first optical measuring device, comprising an alignment microscope, and a further scattered radiation measuring device. A wafer holder is arranged on a positioning device in this lithography system. The US 2008/0 112 609 A1 describes a positioning method and a positioning device using a substrate having alignment marks.

It is an object of the present invention to develop such an optical system such that a positioning of a substrate, which is processed in the lithographic structure generation, in particular can be achieved with good accuracy. In particular, should be possible to find a position as accurate as possible for a structure to be generated on a relatively large to the structure size substrate surface, since the writing field regularly only a very small section an entire substrate surface provides on the exact position a desired structure is to be generated.

This object is achieved by an optical system with the features specified in claim 1.

According to the invention, it has been recognized that positioning of the substrate to be patterned in such a way that the structure to be generated is generated in a write target position is improved with the aid of stored relative coordinates which specify the position of the write field relative to the position of a feedforward field. This takes into account the fact that the writing field is regularly very small, with a much larger Vorschaufeld is used in relation to this. In turn, the feed fence regularly covers only a small part of an entire substrate surface on which the structure is to be produced with exact position. The writing field can then be optimized for the structure-generating process and, in particular, can be made very small. A typical size of the feed fence is 10 x 10 mm ² . A typical size of the writing field is 400 x 400 μm ² . The feedforward field can be greater than the write field by a factor of 50, by a factor of 100, or by an even greater factor, for example by a factor of 1,000 or even 10,000. The entire substrate surface has a typical size of 20 x 20 cm ² . This value applies to a rectangular substrate. A typical round substrate has an area that is about 75% of the area of the rectangular substrate. The writing focus has a typical area of about 1 μm ² .

By using the relative coordinates, a high throughput can be ensured in the structure production. There may be micro and / or nanostructures getting produced. In lithographic patterning with the optical system, maskless or mask based lithography may be used. The preview optics can image the preview field with a magnification in the range between 5 and 100, for example in the range between 30 and 40.

For detecting the Vorschaufeldes the optical system may comprise a digital camera, which may be formed as a CCD camera.

For the structure production one-photon lithography or multi-photon lithography can be used.

A process camera according to claim 2 can be used for determining the relative coordinates of the position of the write field relative to the position of the feedforward field. The process camera can have a chip for spatially resolving the writing field. This may be a CCD chip or a CMOS camera. The writing field can be illuminated by a writing field illumination, which is independent of the writing light beam. The writing field can be illuminated by a wide-field illumination. A light source for the writing field illumination can be designed as an LED. The detection of both independent light for writing field illumination and the writing light on the process camera is advantageous because this observation of a writing process can be done and / or conclusions on a structure structure, in particular a successful polymerization can be drawn. The writing light and / or independent light for writing field illumination can be coupled into the process camera via a beam splitter, in particular via a partially transmissive mirror.

With the aid of a displaceability according to claim 3, an adaptation of focal planes of the projection optics on the one hand relative to the preview optics on the other hand can be made possible. The displacement can be done by means of a corresponding displacement drive. The preview optics and / or the projection optics can be designed to be displaceable in the direction perpendicular to the substrate plane.

By means of an autofocus device according to claim 4, an optimal focal position of the projection optics and / or the preview optics can be determined. The autofocus device can have an illumination that is independent of the writing light or of the preview light.

Image-side numerical apertures according to claims 5 and 6 have proven to be particularly suitable for the function of the projection optics on the one hand and for the function of the preview optics on the other hand. The image-side numerical aperture of the preview optics may be 0.05, for example.

The light source according to claim 7 may be a pulsed light source. The writing light source may be an NIR laser. The writing light source may be an ultrashort pulse laser light source. For the preview optics, a separate from the writing light source light source can be used, which may also be part of the optical system. In the event that the writing field on the one hand and the Vorschaufeld on the other hand do not overlap, is regularly used independent of the writing light source illumination for the preview optics.

A method for determining the relative coordinates according to claim 8 has been found to be particularly efficient. The test object can first be recorded in the preview field with the preview optics and then in the writing field with the projection optics. A reverse order of the shots is possible.

A fine positioning according to claim 9 increases the accuracy in determining the relative coordinates. In the correlation maximization, a rotation and / or a compression and / or a displacement of the test object can take place. In addition to the fine positioning, such a maximization of the correlation can also be used to check and, if necessary, correct a combination of the two optics. For example, the layers of focal planes of both optics to each other can be checked and corrected if necessary. Also, a predetermined magnification ratio of the two optics can be monitored.

An image stack recording according to claim 10 increases an accuracy of a determination of the relative coordinates also along the coordinate perpendicular to the substrate plane. This improves focus positioning in lithographic patterning.

With a multiple raw displacement according to claim 11, a determination of the relative coordinates can take place in the context of an iterative process. This increases the accuracy of the coordinate determination.

A method of lithographic pattern generation according to claim 12 utilizes the advantages of relative coordinate determination using the illustrated optical system.

Aligning the write-destination position in the preview field according to claim 13 enables optimized use of positioning components in writing the predetermined structure in the writing field.

Fig.1

in einer schematischen Seitenansicht Hauptkomponenten eines optischen Systems zur lithografischen Strukturerzeugung; und

Fig.2

ebenfalls schematisch in einer Aufsicht ein Vorschaufeld einer Vorschauoptik des optischen Systems und ein hiervon beabstandetes Schreibfeld, in dem ein Schreibfokus einer Projektionsoptik des optischen Systems angeordnet ist.

An embodiment of the invention will be explained in more detail with reference to the drawing. In this show:

Fig.1

in a schematic side view of main components of an optical system for lithographic structure production; and

Fig.2

also schematically in a plan view a preview field of a preview optics of the optical system and a spaced therefrom writing field in which a writing focus of a projection optics of the optical system is arranged.

An optical system 1, whose main components in the Fig. 1 are shown, serves for lithographic structure generation. A pattern forming method that can be performed with the optical system 1 is explained in more detail in FIG US 2013/0 221 550 A1 , Details of a product resulting as a result of the structure formation are described in U.S. Patent Nos. 5,236,355 US 2013/0 223 788 A1 ,

Part of the optical system 1 is a lithography system 2 with a light source 3 for generating a bundle of writing light 4. An optical path of the writing light 4 is in the Fig. 1 shown very schematically dotted. The light source 3 may be a pulsed NIR (near infrared) laser with a wavelength of 780 nm, for details also in the US 2003/0 221 550 A1 are indicated.

Part of the lithography system 2 is a deflection device 5 for the writing light beam. The deflection device 5 may have one or more tiltable scanning mirrors for deflecting the writing light beam. The deflection device 5 may include at least one optical component, which is designed as a microelectromechanical system (microelectromechanical system, MEMS) component.

The bundle of writing light 4 passes in its course after the light source 3, two output mirrors 6 and 7, which are successively arranged in the beam path of the writing light 4. In the further course, the writing light 4 passes through a projection optics 8 in the form of a microscope objective or in the form of a lithography objective. The projection optics 8 serve to guide the structure-generating pencil of light beams into a writing focus 9 (cf. Fig. 2 ) in the region of a substrate surface 10 of a substrate 11 in a substrate plane 12. A surface within which can be caused by the write focus 9 a used for structure generation polymerization of substrate material, a lateral extent in the range between 1 and 100 microns ² , for example between 2 and 10 μm ² . The writing focus 9 can be approximately round, but can also have an x / y aspect ratio that deviates significantly from 1 and can have, for example, a lateral extent in the range of 1 μm × 10 μm.

A structure is made by displacing the writing focus 9 relative to the substrate material. The structure produced results as the sum of a plurality or a plurality of polymerized dot areas or polymerized lines. For example, a typical such structure may have a lateral extent in the range of 5 μm x 200 μm. The dot area where the polymerization takes place is a volume pixel, So a voxel. The projection optics 8 has a picture-side numerical aperture which is greater than 0.5 and which in the exemplary embodiment is greater than 1.0 and, for example, in the range between 1.2 and 1.4.

The deflection device 5 serves to deflect the writing focus 9 of the writing light beam within a writing field 13 in the substrate plane 12 in the region of the substrate surface 10. Within the entire writing field 13, structure can be generated with the aid of the writing focus 9. For this purpose, the writing focus 9 can be moved in the writing field 13, in particular scanned. A generated structure can thus be significantly larger than the extent of the writing focus 9.

The structure generation in the write focus 9 can be done via a one-photon process or via a multi-photon process.

To the optical system 1 further includes a preview optics 14 for imaging a Vorschaufeldes 15 (see again Fig. 2 ) in the substrate plane 12 in the region of the substrate surface 10. The preview field 15 has an area which is larger by at least a factor 10 than an area of the write field 13. The preview field 15 can be at least a factor 100 larger than the area of the write field Writing field 13.

The fields 13, 15 are in the Fig. 1 not reproduced to scale.

To illustrate positional relationships between the structural elements of the optical system 1, a Cartesian xyz coordinate system is used below. In the side view Fig.1 the xy-plane is perpendicular to the plane of the drawing and coincides with the substrate plane 12. The z-direction is perpendicular to this and runs in the Fig. 1 up. An extent of the fields 13, 15 and a position of the writing focus 9 can thus be specified by x, y coordinates.

The writing field 13 has an xy dimension with a typical edge length in the range between 100 μm and 1 mm, for example an extent of 400 μm × 400 μm. A focus diameter of the writing focus 9 in a focal plane of the writing light beam is in a range between 0.25 μm and 50 μm. A typical focus diameter of the writing focus 9 is 1 μm. The writing field 13 is therefore typically greater than the diameter of the writing focus 9 by more than a factor 100 along each of the two coordinates x and y.

The Vorschaufeld 15 has an extension with a typical edge length between 1 mm and 50 mm, for example, a typical xy-dimension of 10 mm x 10 mm. The Vorschaufeld 15 may be even larger and have a typical xy-expansion, for example, 50 mm x 50 mm. The Vorschaufeld 15 is smaller than the entire substrate surface 10, which may have an area of, for example 20 x 20 cm ² . The substrate 11 may be rectangular or round. The area of the round substrate is typically a factor of 0.75 smaller than that of the rectangular substrate.

The preview optics 14 increases the preview field 15 with a magnification in the range between 5 and 100, typically between 30 and 40. A working distance between a substrate-next component of the preview optics 14 and the substrate 10 is in the range between 1 mm and 30 mm and is typically 10 mm ,

The preview optics 14 has a picture-side numerical aperture which is smaller than 0.1 and which may be, for example, 0.05.

The projection optics 8 and the preview optics 14 are supported by a common frame 16, which in the Fig. 1 is indicated schematically and broken. The projection optics 8 or the preview optics 14 are rigidly connected to one another via a frame carrier 17 with respect to the coordinates x and y.

The substrate 11 is supported by a substrate holder 18. This can be displaced in the xy plane in the two translational degrees of freedom x and y. For this purpose, the substrate holder 18 is connected to an xy displacement drive 19. The substrate holder 18 with the xy displacement drive 19 can be designed as xy table with a positioning repeatability better than 5 microns.

For the detection of the Vorschaufeldes 15, the preview optics 14 on a CCD camera 20. Alternatively or additionally, the preview optics 14 may include a tube for visual inspection via an operator. An illumination of the Vorschaufeldes 15 via an integrated into the preview optics 14 preview light source 21, which is independent of the writing light source 3. The preview light source 21 generates preview light 22 for illuminating the Vorschaufeldes 15. The beam path of the preview light 22 is in the Fig. 1 also indicated very schematically dotted. The preview light source 21 may be a light source in the visible wavelength range. Alternatively, it is possible in principle to use the writing light source 3 also for generating preview light for illuminating the preview field 15.

The preview light source may provide wide field illumination and / or structured illumination. Here, a speckle pattern or a stripe projection can be used.

The optical system 1 further comprises a process camera 23 for detecting the writing field 13. This is done using a separate illumination of the writing field, which is not shown in detail in the drawing. Alternatively or additionally, for detecting at least part of the writing field 13, the beam path of the writing light beam in the projection optics 8 can be used. From the writing pad 13 in the projection optics 8 retroreflected light, z. As the writing light 4 is coupled out of the partially transparent Auskoppelspiegel 7 in the process camera 23. The process camera 23 has a CCD chip 24 for detecting the light coupled out via the output mirror 7 light 4a. If, for the detection of the writing field 13, a lighting independent of the writing light source is used, the writing light 4 can nevertheless be additionally detected via the process camera 23.

Furthermore, the optical system 1 comprises an autofocus device 25 for determining a focal plane of the projection optics 8 and / or the preview optics 14. The autofocus device 25 detects a further light bundle 4b coupled out via the partially transparent output mirror 6. The autofocus device 25 may have its own illumination, which is not shown in detail in the drawing.

The projection optics 8 can be displaced relative to the preview optics 14 in the z-direction, ie perpendicular to the substrate plane 12. For this purpose, the projection optics 8 is equipped with a z-displacement drive 26. The z-displacement drive 26 is in the Fig. 1 indicated schematically and is between the Frame carrier 17 and the projection optics 8 are arranged. In this z-displacement of the projection optics 8 relative to the preview optics 14 is a relative movement of the two optics 8, 14 guided to each other via a z-guide. This z-guide is part of the frame carrier 17th

An adjustment of the focal planes of the projection optics 8 and the preview optics 14 can take place with the aid of the z displacement drive 26.

The optical system 1 furthermore has a control unit 27. The control unit 27 has a memory 28 in which relative coordinates (RK _x , RK _y ) of the position of the writing field 13 are stored relative to the position of the preview field 15.

The optical system 1 can be used to lithographically produce microstructures and / or nanostructures. This can be done maskless, ie without imaging an object structure on the substrate surface 10, but alternatively also mask-based.

To determine the relative coordinates RK _x , RK _{y of} the position of the writing field 13 relative to the position of the Vorschaufeldes 15 proceeds as follows:
First, a test object, for example a rectangular structure with a predetermined x / y aspect ratio, is recorded with the preview optics 14. The position of the test object in the preview field 15 is detected by means of the CCD camera 20. Subsequently, the test object is positioned in the preview field 15 by means of the xy displacement drive 19, so that the test object in the preview field 15 has a defined position. This position can be selected, for example, such that a mark on the test object on the central coordinates x _v , y _v (cf. Fig. 2 ) of the Vorschaufeldes 15 comes to rest.

At the same time, it can be ensured by controlled actuation of the z-displacement drive 26 that the test object lies optimally in an image-side focal plane of the preview optics 14.

Subsequently, the test object is rough-shifted by means of the xy displacement drive 19 on the substrate holder 18 between the feed fence 15 and the writing pad 13. In this raw displacement raw relative coordinates RK _{x, raw} ; RK _{y, raw} documented and stored in memory 28. The aim of the raw relocation is to move the test object into the writing field 13.

Subsequently, the test object with the projection optics 8 and the process camera 23 is recorded. Subsequently, the test object is finely positioned in the writing field 13 until the test object in the writing field 13 has a defined position. This defined position can, for example, be such that the marking of the test object after fine positioning in the writing field 13 has a center x _s , y _s (cf. Fig. 2 ) of the writing field 13 matches. The changes in the raw relative coordinates of the test object during the fine positioning are also documented and stored in the memory 28. From the raw relative coordinates and the documented changes in the fine positioning, the desired relative coordinates of the position of the writing field 13 relative to the position of the preview field 15 can then be documented and stored in the memory 28. For example Fig. 2 the following relationship between the relative coordinates RK _x , RK _y and the coordinates x _v , y _v and x _s , y _{s of} the fields 15 and 13 results: $${\mathrm{RK}}_{\mathrm{x}}.{\mathrm{RK}}_{\mathrm{y}}=\left({\mathrm{x}}_{\mathrm{v}}-{\mathrm{x}}_{\mathrm{s}}\right).\left({\mathrm{y}}_{\mathrm{v}}-{\mathrm{y}}_{\mathrm{s}}\right)$$

The generated relative coordinates RK _x , RK _y are then stored in the memory 28 of the control unit 27 and are available for a subsequent retrieval when moving a substrate on the lithographically a structure to be generated in a write-destination position.

After the determination of the relative coordinates RK _x , RK _y , the preview optics 14 are calibrated relative to the position optics 8. A displacement of the substrate holder 18 about the coordinates (RK _x , RK _y ) transfers an object carried by the substrate holder from the center of the preview field 15 into the center of the writing field 13.

By knowing the extent of the preview field 15, a position selected by the user in the preview field 15 can also be transferred directly to a center in the writing field 13. This is done by the control unit 27 initially centered on this selected position in Vorschaufeld 15, which is done by appropriate positioning on the xy displacement drive 19 and documenting corresponding relative coordinates. Subsequently, the transfer transfer described above is performed. The centering and transfer relocation can also be combined within one step.

Alternatively, the determination method described above can also be carried out starting from a recording of the test object in the write field 13, wherein the test object is first positioned in the writing field 13 and then a raw displacement of the test object from the writing field 13 into the preview field 15 with corresponding fine positioning takes place within the Vorschaufeldes 15. The documentation first of the raw relative coordinates and the generation thereof and the result of the fine positioning of the desired relative coordinates then takes place analogously to what has already been explained above in connection with the determination method, starting from the test object in the preview field 15.

Depending on the order of positioning of the test object, the output field is thus either the preview field 15 or the writing field 13 and the target field is either the Schreifeld 13 or the Vorschaufeld 15th

In fine positioning, a correlation maximization of the positions of the test object in the output field on the one hand and in the target field on the other hand takes place. This correlation is maximized by rotation of the test object and / or by compression of the test object and / or by displacement of the test object.

The rotation takes place about an axis parallel to the z-axis, which can be done by an additional pivot motor 29 of the substrate holder 18. Alternatively, a rotation can also be done by an image rotation of the respective detection optics in the representation for the user. To detect such a rotation, a plurality of markings are applied to different x, y positions of the test object. After raw relocation or fine positioning, rotation of these test marks with target marks can be maximized by rotation.

During the compression, a magnification ratio between the production optics 8 and the preview optics is produced via the z displacement drive 26 14 varies until a predetermined ratio coincides with a distance ratio of test marks on the test object during the recording on the one hand via the preview optics 14 and on the other hand via the projection optics 8.

The correlation maximization by displacement, ie by translation in the degrees of freedom x and y, has been described above in connection with the Fig. 2 already explained.

In the test object recording, an image stack can be obtained by recording different images of the test object with the respective imaging optics, ie with the projection optics 8 or the preview optics 14 in different z-displacement positions of this recording optics relative to the test object in the z-displacement direction perpendicular to xy substrate plane 12 are generated.

In addition to the relative coordinates along the x-coordinate and along the y-coordinate, a relative coordinate in the z-direction can also be stored if there is a z-offset of the focal planes between focal planes of the projection optics 8 on the one hand and the preview optics 14 on the other hand.

When determining the relative coordinates, a raw displacement of the test object between the fields 13, 15 can take place several times. The determination of the relative coordinates can then take place within the scope of an iterative process.

For lithographic structure generation with the optical system 1, the relative coordinates of the position of the writing field 13 relative to the position of the advance field 15 are initially determined, as explained above. Subsequently For example, the substrate 11 is provided on the substrate holder 18. It can then be driven on the entire substrate 11 under the Vorschaufeld 15 by means of the xy displacement drive 19 a provided for the structuring portion. A write destination position on the substrate 11 in the preview field 15 is then identified and selected by the user. This write target position can then be aligned in the preview field 15. Subsequently, the write target position is shifted from the preview field 15 into the write field 13 using the determined relative coordinates RK _x , RK _y . Then, a predetermined structure is written in the writing field 13 with the writing light beam, whereby the writing focus 9 is displaced according to the shape of the predetermined structure by means of the deflecting device 5 in the writing field 13.

As the substrate 11, a wafer having a diameter of, for example, about 15 cm (6 inches) or about 20 cm (8 inches) may be used. Even a wafer with an even larger diameter can be used.

Claims (13)

1. Optical system (1) for producing lithographic structures,

   - comprising a projection optical unit (8) for guiding a structure-producing writing light beam into a writing focus (9) in the region of a substrate surface (10) in a substrate plane (12),

   - comprising a deflection device (5) for deflecting the writing focus (9) of the writing light beam within a writing field (13) in the region of the substrate surface (10),

   - comprising a preview optical unit (14) for imaging a preview field (15) in the region of the substrate surface (10), wherein the preview field (15) has an area which is greater than an area of the writing field (13) by at least a factor of 10,

   - wherein the projection optical unit (8) and the preview optical unit (14) are carried by a common frame (16),

   - comprising a substrate holder (18), which is displaceable in a plane (xy) parallel to the substrate surface (10) with two degrees of translational freedom (x,y),

   - comprising a control unit (27) comprising a memory (28), in which relative coordinates (RK_x, RK_y) of a position of the writing field (13) relative to the position of the preview field (15) are stored.
2. Optical system according to Claim 1, characterized by a process camera (23) for capturing the writing field (13) using the beam path of the writing light beam in the projection optical unit (8).
3. Optical system according to Claim 1 or 2, characterized in that the projection optical unit (8) is displaceable relative to the preview optical unit (14) in a direction (z) perpendicular to the substrate plane (12).
4. Optical system according to any one of Claims 1 to 3, characterized by an autofocusing device (25) for determining a focal plane of the projection optical unit (8) and/or of the preview optical unit (14).
5. Optical system according to any one of Claims 1 to 4, characterized in that the projection optical unit (8) has an image-side numerical aperture which is greater than 1.0.
6. Optical system according to any one of Claims 1 to 5, characterized in that the preview optical unit (14) has an image-side numerical aperture which is less than 0.1.
7. Optical system according to any one of Claims 1 to 6, characterized by a light source (3) for producing writing light (4).
8. Method for determining relative coordinates (RK_x, RK_y) of a position of a writing field (13) relative to a position of a preview field (15) in an optical system (1) according to any one of claims 1 to 7, said method comprising the following steps:

   - recording a test object using one of the two optical units (8, 14) from the optical group: preview optical unit (14) and projection optical unit (8) in an initial field (13, 15), namely in one of the two fields from the field group: preview field (15) and writing field (13),

   - positioning the test object in the initial field (13, 15) such that the test object has a defined position in the initial field (13, 15),

   - carrying out, by way of the substrate holder (18), a coarse displacement of the test object between the initial field (13, 15) and a target field (15, 13), namely the other of the two fields from the field group,

   - registering coarse relative coordinates (RK_{x, coarse}; RK_{y, coarse}) of this coarse displacement,

   - recording the test object using the other of the two optical units (14, 8) from the optical group,

   - finely positioning the test object in the target field (15, 13) such that the test object has a defined position in the target field (15, 13),

   - registering changes in the coarse relative coordinates (RK_{x, coarse}; RK_{y, coarse}) during the fine positioning and producing the relative coordinates (RK_x, RK_y) from the coarse relative coordinates (RK_{x, coarse}; RK_{y, coarse}) and the result of the registration.
9. Method according to Claim 8, characterized in that a correlation of the position of the test object in the initial field (13, 15) on the one hand and in the target field (15, 13) on the other hand is maximized during the fine positioning.
10. Method according to Claim 8 or 9, characterized in that an image stack is produced during the test object recording by recording various images of the test object using the recording optical unit (8, 14) in various displacement positions of the recording optical unit (14, 8) relative to the test object in a displacement direction (z) perpendicular to the substrate plane (12).
11. Method according to any one of Claims 8 to 10, characterized in that a coarse displacement of the test object between the fields (13, 15) of the field group occurs multiple times during the determination.
12. Method for producing lithographic structures using the optical system (1) according to any one of Claims 1 to 7, said method comprising the following steps:

    - determining the relative coordinates (RK_x; RK_y) of the position of the writing field (13) relative to the position of the preview field (15) using the method according to any one of Claims 8 to 11,

    - providing the substrate (11) on the substrate holder (18),

    - determining a writing target position on the substrate (11) in the preview field (15),

    - displacing the writing target position from the preview field (15) into the writing field (13) using the determined relative coordinates (RK_x, RK_y),

    - writing a predetermined structure into the writing field (13) using the writing light beam.
13. Method according to Claim 12, characterized in that the writing target position is aligned in the preview field (15) before the writing target position is displaced from the preview field (15) into the writing field (13).

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